1. Field of the Invention
The invention relates generally to the interconnect between the read/write circuitry and the read/write head in a magnetic recording hard disk drive (HDD), and more particularly to an interconnect with a transmission line optimized for overshoot of the write current pulses.
2. Description of the Related Art
In magnetic recording HDDs, the read/write head is formed on an air-bearing slider that rides on a thin film of air above the rotating disk. A mechanical suspension comprised of a flexure with a gimbal at its end connects the slider to the disk drive's actuator arm. The slider is attached to the gimbal which allows for slight movement on the air bearing as the disk rotates. A transmission line provides electrical connection from the read/write circuitry, typically in a read pre-amplifier/write driver module or chip, through a short flex cable and suspension to the read and write elements on the slider. A suspension that integrates the mechanical connection with the electrical connection is called an integrated lead suspension (ILS) that is connected between the flex cable and connection pads on the slider. A typical ILS is a generally flexible laminate of a conductive metal substrate like stainless steel, an insulating dielectric layer like polyimide, and electrically conductive copper lines or traces patterned on the dielectric layer. The transmission line for the signals from the write driver to the write head thus include electrically conductive traces on the flex cable and the ILS.
The write driver circuitry typically provides a single-point input to the transmission line for each of the positive and negative write signals (+W and −W). The write driver circuitry provides current through the transmission line to the write element or head. The write driver circuitry power supply voltage and performance of the current through the write head depends on the characteristic impedance of the transmission line because the launch voltage at the write driver's output directly depends on the write-current signal times the characteristic impedance.
In current HDDs, boosts or overshoot of the write current pulses to the write head are used to overcome the relatively slow magnetic response of the head and media, i.e., the magnetic recording layer on the disk. Thus, the overshoot is required to create the precise magnetic write field profile with sharp field gradient, i.e., the correct magnetic “footprint”, in the disk media. The lack of a precise magnetic footprint can cause problems at low and high data rates. Therefore, the overshoot of the write current pulses is typically required at all times. The present solution to achieving fast magnetic flux reversals is to use electronics in the write driver circuitry to create write current overshoot. However, the use of electronics can be a limiting factor as data rates are increased and power supply voltages are reduced, which limits the write driver's current drive capability. Another approach for achieving faster magnetic flux reversals uses transmission line reflections. This inherent transmission line overshoot approach reduces the overshoot requirement from the write driver circuitry.
What is needed is a HDD with a write driver to write head transmission line with a characteristic impedance that provides optimal overshoot for the write current pulses.